The present invention relates to a method and to a device for protecting a transmission system for protecting the system against line and board failures.
It is necessary to ensure in transmission systems, such as telecommunications systems, that the signals transmitted in the system will arrive at the receiver, even though a standard line may be broken or a receiving or transmitting unit is out of operation. A system operator accepts no losses of signals in the transmission system, and consequently it is necessary to take precautionary measures to ensure that the system will function faultlessly even when such faults occur A number of devices and methods are known to this end.
Patent Specifications U.S. Pat. No. 5,365,510, U.S. Pat. No. 5,179,548 and EP-A1 677 936 teach different methods of handling cable breakages in optical transmission systems. Briefly, the protective systems include an additional line in the form of an optical fibre which is connected between the different nodes in the system, in the same manner as the standard line but separated physically therefrom. Communication on the spare line takes place when the standard line has broken at some place or other. In an acute case, when the transmitter unit or receiver unit is out of function, a so-called head is formed on one side of the node and a so-called tail is formed on the other side of the node, therewith disconnecting the node from the network, such that communication will continue between the remaining nodes in the network.
It is necessary to provide in transmission systems, and then particularly in telecommunications systems, some form of protective device and some form of protective procedure that will ensure that signals transmitted in the system will always reach their intended receiver, irrespective of whether a line breakage occurs or if a transmitter/receiver unit stops functioning.
The present invention addresses a problem as to how this protective device can be implemented in the transmission system in the cheapest and simplest possible manner, and how the combined protection against a line fault and against a board fault can be incorporated in one and the same device.
The aforesaid problem is solved by means of the present invention in that the boards which form the interface against outgoing and incoming lines, the so-called line boards, include a protective switch and are mounted in a protective ring together with a stand-by unit.
Each line board includes a line driver, a line receiver and a number of switches. Each line board includes an associated incoming and outgoing line. These boards include the following interfaces:
an interface against its corresponding connection board, a so-called connection interface;
the interface against incoming and outgoing lines, the so-called line interface; and
an interface against the protective ring, a so-called protective ring interface.
As before mentioned, the protective ring comprises said collection of line boards to be protected, and a stand-by unit which is actuated in the event of a line fault or in the event of a connection board fault. Switching to the stand-by unit can be controlled by a central monitoring unit mounted in a magazine or in the stand-by unit.
The stand-by unit includes a stand-by line board and a stand-by connection board. A connection board forms the interface against the switching centre or telephone exchange unit in the transmission system and thus includes an interface against this switching centre, the so-called switching centre interface, and a further interface against the line board, the aforementioned connection interface.
When a fault occurs in the transmission system, the line driver will detect the loss of signal, LOS, and can change the states of the switches in the protection switch in coaction with the central unit, such as to switch an appropriate part of the traffic to the stand-by unit.
The protection ring may include means for transmitting a pilot tone or some equivalent signal from the stand-by connection board, for monitoring the protection ring.
Thus, the intention of the present invention is to combine protection against line faults and board faults in one single device that includes the protection ring and the protection switches.
The present invention has the advantage of being user-friendly and flexible.
Another advantage afforded by the present invention is that it does not require the use of one card to remedy a line fault and another card to remedy a board fault in one and the same transmission system.
Another advantage is that the present invention does not require a spare line for each line board.
Still another advantage is that each line board in the protection ring can be a stand-by line board.
Yet another advantage is that the protection ring can be monitored via a so-called pilot tone that will not interfere with traffic in the system.